This invention relates to well logging and, more particularly, to a method and apparatus for determining more accurately the dielectric permittivity, conductivity, and water-filled porosity of subsurface formations to obtain improved well logs.
It has long been recognized in the well logging art that a knowledge of the dielectric permittivity of a formation surrounding a borehole would provide useful information concerning the nature of fluids in the formation, since there is a striking contrast between the dielectric constant of water (about 80) and the dielectric constant of hydrocarbons (about 5). The development of electromagnetic propagation types of devices, such as the electromagnetic propagation tool ("EPT" device) described in U.S. Pat. No. 3,944,910, provided an improved technique for determining formation dielectric permittivity, as well as conductivity, using microwave electromagnetic energy at shallow depths of investigation.
Another device which measures formation dielectric permittivity and conductivity is known as a deep propagation tool ("DPT" device), an embodiment of which is described in U.S. Pat. No. 4,209,747. This device operates using radio frequency electromagnetic energy, and is capable of deeper investigation into the formations.
As described further hereinbelow, the measurements of dielectric permittivity and conductivity taken at practical measurement frequencies are found to include components which tend to make the measurements less useful. It is among the objects of the present invention to provide techniques for obtaining improved measurements of formation dielectric permittivity and conductivity, and to also obtain improved measurements of formation water-filled porosity.